1. Field of the Invention
The present invention relates to a method for manufacturing a hot-dip plated metal strip and an apparatus for manufacturing thereof.
2. Description of Related Arts
Hot-dip plating is a known method of continuous plating for a metal strip such as steel strip, which hot-dip plating method conducts metal strip plating by immersing the metal strip in a bath of molten metal of plating metal such as zinc and aluminum, (hereinafter referred to simply as “molten metal bath”). The hot-dip plating method has many advantages such as allowing manufacturing a plated steel strip at low cost compared with an electroplating method and allowing easily manufacturing a plated metal strip with thick coating layer.
FIG. 1 shows a conventional manufacturing line of hot-dip plated metal strip.
The metal strip 1 which was rolled in the preceding step of cold-rolling and was cleaned on the surface thereof in the succeeding cleaning step is transferred to a hot-dip plated metal strip manufacturing line, where the surface oxide film is removed and the metal strip is annealed in an annealing furnace 71 which is maintained in non-oxidizing or reducing atmosphere. Then, the metal strip 1 is cooled to a temperature almost equal with the temperature of a molten metal bath 2, and is introduced to the molten metal bath 2, where the molten metal is adhered onto the surface of the metal strip 1. After that, the metal strip 1 is taken out from the molten metal bath 2, and a gas ejected from a gas wiper 6 removes excess amount of molten metal adhered to the metal strip 1 to adjust the plating weight of the molten metal, thus to form the plating layer of the molten metal onto the metal strip 1.
As shown in FIG. 2, the metal strip 1 is introduced to the molten metal bath 2 via a cylinder 4 called “snout” which is kept to a non-oxidizing atmosphere therein, and the metal strip 1 is turned the running direction in the molten metal bath 2 by a sink roll 3 therein. Before being taken out from the molten metal bath 2, the metal strip 1 is corrected in the warp generated in width direction thereof and suppressed in the vibration thereof by a stabilizing roll 79a and a correct roll 79b, (both rolls are collectively called “submersed support rolls 79”).
The metal strip 1 coated with a plating layer is subjected to various treatments depending on the uses thereof to become a final product. For example, when the metal strip 1 is used as an external panel of automobile, the metal strip 1 is subjected to alloying treatment of plating layer in an alloying furnace 9, and is introduced to a quenching zone 75, then is subjected to special rust-preventive and corrosion-preventive treatment in a conversion treatment unit 76.
The hot-dip plating method, however, has problems described below.
1) An impurity called “dross” is generated in the molten metal bath 2, which dross adheres to the metal strip 1 and to the submersed support rolls 79 to become a defect of the metal strip 1 reducing the yield thereof. To this point, high grade hot-dip plated metal strip used in, for example, an automobile external panel is processed at a low speed operation to prevent the adhesion of dross. The countermeasure, however, significantly degrades the productivity.
2) Since the submersed support rolls 79 are exposed to severe environment of high temperatures, troubles such as insufficient rotation likely occur, so that regular shut down of the line is requested for maintenance and replacement of the rolls, which degrades the productivity. In addition, these troubles may cause defects such as dross adhesion to the metal strip 1.
3) Owing to irregular rotational speed of the submersed support rolls 79, irregular plating weight occurs to induce chatter marks, which degrades the product quality.